A conventional optical module of this type has been disclosed in Japanese patent No. 2719804, for example. This conventional optical module comprises, as shown in FIG. 1, a planar microlens array 60 consisting of a planar transparent substrate having circular microlenses 61 formed in a surface thereof. Fitting recesses 65 are formed in a surface opposite to the lens-formed surface of the array 60, each of recesses 65 being aligned with the center of a corresponding microlens 61. An optical element to be optically coupled to the microlens 61 is an optical fiber 63, for example. The end core portion of an optical fiber is processed by a selective etching to form a micro fitting convex portion 66. According to the conventional optical module described above, an alignment can easily be conducted by inserting the convex portion 66 of an optical fiber into the fitting recess 65 to fix it thereto, instead of an active alignment (i.e., light is guided into an optical fiber and the position of the optical fiber is regulated so as to maximize light coupled to a microlens). In FIG. 1, the light from the end of the optical fiber 63 is transferred into collimated light 100 by means of the microlens 61.
According to the conventional optical module, there is a problem such that a fabrication of an optical module is complicated because fitting recesses are required for a planar microlens array thereby fixing optical fibers, and a fitting convex portion is required at the end of an optical fiber.
Furthermore, a step for fabricating a resin layer having guide holes through which an end of an optical fiber is inserted is required. In the technique disclosed in described-above Japanese Patent No. 2719804, while guide holes are formed in a resin layer by a lithography process, the process is very complicated.
There is another method for fixing a guide substrate for optical fibers to a planar microlens array in which guide holes have been opened. In this case, a machining such as a laser beam machining, a drilling, or an ultrasonic machining is used as a method for opening holes.
While a laser beam machining is a relatively low cost method, it has problems such that the control of a diameter of a hole is difficult resulting in a low circular degree of a hole, and furthermore the taper of a guide hole is not preferably controlled. In addition, the kind of a material for a substrate to be processed is limited. Also, a backing to a substrate is required to prevent a chipping (i.e., a partial peeling of a substrate) from being caused on a bottom side of a substrate. However, a chipping may not be completely prevented for a backed substrate. An adhering, separating and cleaning steps are further required for backing a substrate.
It is also difficult for a machining to precisely open a hole the diameter thereof is on the order of 125 μm.
In an aligning method using a guide substrate for optical fibers, an aligning process between a planar microlens array and a guide substrate for optical fibers is required during the assembling of an optical module. In order to prevent misalignment between an optical fiber and a microlens due to a temperature change, the coefficient of thermal expansion of a microlens array substrate and that of a guide substrate for optical fibers must be the same.